1. Field of the Invention
The present invention relates to an optical disk quality inspection apparatus, and more particularly, to an improved apparatus and method for optical disk quality inspection capable of effectively inspecting disk quality during the disk fabrication process.
2. Description of Related Art
Disk media are divided into two groups, magnetic disks and optical disks such as compact disks (CDs). Because of their large storage capacity, the demand for optical disks has rapidly increased. Optical disks are classified into a pre-recorded disk such as a compact disk (CD), a write-once disk such as CD-R (CD-recordable), and a rewritable disk medium such as CD-RW and DVD-RAM.
FIGS. 1A and 1B illustrate conventional optical disks. The radius L1 of these disks is 60 mm, while the diameter L2 of the center hole is 15 mm. FIG. 1A illustrates a conventional prerecorded optical disk. As shown in FIG. 1A, a recording area of the optical disk includes a lead-in start point BLI, a main storage area start point BPL, and a lead-out start point BLO. Information is written in the main storage area MSA between the main storage area start point BPL and lead-out start point BLO.
FIG. 1B illustrates a conventional write-once optical disk upon which no information has been recorded. As with the optical disk of FIG. 1A, the optical disk of FIG. 1B has a recording area, and when information is recorded thereon, this information is recorded in a main storage area MSA. As shown in both FIGS. 1A and 1B, an outer area of the recording area is not used to store information. This provides a margin of error with respect to manufacturing defects since defects in the recording area are much more likely to occur in the outer area of the recording area due to the manufacturing techniques used.
FIGS. 2A and 2B are schematic views illustrating the architecture of a CD-R disk.
Initially, a CD-R disk is injection-molded using a stamper to form a pre-grooved polycarbonate substrate. Then, a spin coating process is applied to the fabrication of the pre-grooved polycarbonate substrate.
As shown in FIG. 2A, the spin coating process allows the pre-grooved polycarbonate substrate to rotate at about 3000 rpm so as to carry out a disk coating process, wherein a coating liquid is dropped inward the disk so as to be spread out in accordance with its centrifugal force.
The recording layer is initially formed using polyvinyl and the spin coating process, and a reflection layer and a protection layer are sequentially formed using aluminum on the recording layer. A coating layer is formed using UV resin on the protection layer, and then finally a labeling layer is formed to thereby obtain a CD-R disk having a cross-section as shown in FIG. 2B. The protection-coated CD-R disk is stacked on a spindle and transferred to a quality inspection apparatus.
As illustrated in FIG. 2A, when the spin coating process is used in optical disk fabrication, the thickness of the disk tends to be gradually thinner toward its outer periphery. As a result, when recording on or playing back from a recording layer of the disk, the occurrence of errors becomes higher toward the outer circumference because of a non-uniformity of a record layer, so that, conventionally, target information is recorded only on a main storage area which accounts for 118 mm out of the entire 120 mm diameter of an optical disk. The outer area beyond the boundary of 118 mm is not employed for recording.
The conventional disk quality inspection methods at large vary from a visual inspection method to a reflection rate inspection method.
FIG. 3 is a block diagram of a visual inspection apparatus manufactured by the Koch Company. As shown therein, the visual inspection apparatus includes a controller 10 for carrying out an overall control operation of the inspection apparatus, drives 11-14 for executing a recording/playingback operation of a test signal on/from a lead-in area of a main storage area, a test signal processing unit 15 for analyzing attributes of test signals played back from the drives 11-14, a measurement system 15 for comparing a reference signal to respective attributes analyzed from the test signal processing unit 15 and determining whether the disk quality is poor.
At this time, it is assumed that the disk is injection-molded using the identical stamper, and four drives are normally adopted with a slight difference depending upon the manufacturer of the measurement apparatus.
The test signal processing unit 15 includes a central processing unit 15a, and digital signal processing units 15b-15e which respectively correspond to the drives 11-14.
Here, DSP1-DSP4 (15b-15e) respectively serve to analyze attributes from test signals played back. The DSP-1 (15b) analyzes a servo signal and a focusing signal, the DSP-2 (15-C) analyzes jitter, DSP3 (15d) analyzes a mechanical attribute, and DSP4 (153) analyzes an optical attribute.
The thusly constituted disk visual inspection apparatus will now be described.
When disk fabrication is completed, disks sampled according to the control of the controller 10 are appropriately mounted in the drives 11-14, and the respective drives 11-14 record/play back the test signals on/from the lead-in area or the main storage area in accordance with the control of the controller 10.
The DSP1-DSP4 15a-15e correspondingly receive the test signals played back from the drives 11-14, and then according to the control of the CPU 15a, analyze a high frequency HF, a jitter, a servo signal, a focusing signal, a mechanical characteristic and an optical characteristic. At this time, the measurement factors include optical characteristics such as double refraction, reflectivity, and permeability, and signal attributes such as error occurrence per block, radical noise and jitter, which are respectively employed to evaluate the disks.
Consequently, the measurement system 16 compares a previously stored reference signal with high frequency, jitter, servo signal, focusing signal, mechanical attribute and optical attribute so as to determine whether the disk quality is poor.
As discussed above, conventional quality test methods involve recording signals in the lead-in area or main storage area of the disks, reproducing those test signals, and comparing the test signals to reference signals to determine quality.
Unfortunately, however, these methods cannot be applied to every optical disk manufactured. For instance, once test data is written into a write-once optical disk, that disk loses its value as a commercial good because it becomes unusable. Accordingly, samples from a plurality of manufactured optical disks are taken and tested according to the above-described methodology.
Besides rendering the sampled disks unusable, these quality testing techniques also prove to be inaccurate and unreliable. Just because the sampled disks may be of sufficient quality does not necessarily mean that the other disks, not sampled, are of sufficiently high quality. Therefore, these tests tend to be inaccurate and unreliable.
Furthermore, in the conventional reflectivity inspection method, the light coming out from a laser diode is branched into a plurality of beams and beamed on the disks. The plurality of beams reflected from the reflection layer of the disk are detected using a plurality of optical detectors, and defects, resulting from a non-uniformity of layers which occur during molding or fabrication (e.g., a sputtering process and a spin coating process), are detected. The accuracy of defect judgement for reflectivity inspection depends on the number of optical detectors and the size thereof.
Currently available technology experiences a limitation in decreasing the size of optical detectors such that only noticeable detects, not fine detects, can be detected.
Optical disk media also undergo additional quality tests. For instance, the entire surface of the optical disk is displayed by monitoring the scanning of the optical disk surface with a laser beam using a CCD camera. The surface is then visually checked using the display.
Other tests include push/pull, crosstalk, and checking the location of the lead-in start point BLI, main storage area start point BPL, and lead-out start point BLO.
Through the above described test process, mechanical characteristic measurements such as the lead-in start point BLI, the main storage area start point BPL, the lead-out start point BLO, a track and index start point, a test speed, a track pitch, a bow deflection, a warp deflection, a thickness of the optical disk, an angular deflection, a vertical deflection, the radius of the optical disk, and the diameter of the center hole of the optical disk are checked. In addition, signal characteristics such as radial noise, focal noise, push/pull of a tracking signal, data carrier analog, and carrier digital are checked.